Hot press-formed part, and manufacturing method thereof

ABSTRACT

The present disclosure provides a hot-press formed part comprising a plated steel sheet and an aluminum alloy plated layer formed on the plated steel sheet, wherein the aluminum alloy plated layer comprises: an alloying layer (I) formed on the plated steel sheet and containing, by weight %, 5-30% of Al; an alloying layer (II) formed on the alloying layer (I) and containing, by weight %, 30 to 60% of Al; an alloying layer (III) formed on the alloying layer (II) and containing, by weight %, 20-50% of Al and 5-20% of Si; and an alloying layer (IV) formed continuously or discontinuously on at least a part of the surface of the alloying layer (III), and containing 30-60% of Al, wherein the rate of the alloying layer (III) exposed on the outermost surface of the aluminum alloy plated layer is 10% or more.

CROSS REFERENCE

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/KR2019/015951 filed on 20 Nov. 2019,which claims the benefit of Korean Application No. 10-2018-0153165 filedon 30 Nov. 2018, the entire contents of each are hereby incorporated byreference.

TECHNICAL FIELD

The present disclosure relates to a hot-press formed member and a methodof manufacturing the same.

BACKGROUND ART

Recently, due to recent depletion of petroleum energy resources and highinterest in the environment, regulations on improving fuel efficiency ofautomobiles have been strengthened. In terms of materials, one method toimprove fuel efficiency of automobiles may be to reduce a thickness of asteel sheet used in automobiles. However, when a thickness is reduced,the safety of automobile may be affected. Thus, strength of a steelsheet may also need to improve.

For this reason, there has been continuous demand for a high-strengthsteel sheet, and various types of steel sheets have been developed.However, since such steel sheets have high strength, workability may bepoor, which may be problematic. In other words, since a product ofstrength and elongation for each grade of a steel sheet may have atendency to have a constant value, when strength of the steel sheetincreases, elongation, an index of workability, may decrease, which maybe problematic.

To solve this problem, a hot-press forming method has been suggested.The hot-press forming method is a method of increasing strength of afinal product by processing a steel sheet at a high temperature suitablefor processing the steel sheet, rapidly cooling the steel sheet to a lowtemperature, thereby forming a low-temperature structure such asmartensite in the steel sheet. In this case, there may be an advantagein that the problem in workability when manufacturing a member havinghigh strength may be reduced.

However, in the case of the hot-press forming method, since the steelsheet is heated to a high temperature, a surface of the steel sheet maybe oxidized, and accordingly, a process of removing oxide on the surfaceof the steel sheet may need to be added after press forming, which maybe problematic. As a method to solve this problem, the techniquedisclosed in U.S. Pat. No. 6,296,805 has been suggested. In the aboveU.S. Pat. No. 6,296,805, a steel sheet having gone through aluminumplating may be used for hot-press forming or room-temperature forming,and heating and rapid cooling (“heat treatment”). Since the aluminumplated layer is present on the surface of the steel sheet, the steelsheet may not be oxidized during heating.

However, in the case of hot-press forming a steel sheet having gonethrough aluminum plating, there may be a problem in that, althoughstrength of a material is extremely low due to high temperature duringhot forming, abrasion of a die may be severe, which may be a problem.That is because, base iron may diffuse into an aluminum plated layerwhile a plated steel sheet is heated for hot forming, and accordingly, ahard Fe and Al alloy layer may be formed on the surface of the steelsheet, and hardness of the alloy layer may be higher than hardness of adie material generally formed of tool steel, such that abrasion of thedie may become severe by press forming. Accordingly, in the case ofhot-press forming a steel sheet having gone through aluminum plating, adie may need to be ground or replaced in a short cycle, which mayincrease manufacturing costs of a hot-press formed member, which may beproblematic.

DISCLOSURE Technical Problem

The purpose of the present disclosure is to provide a hot-press formedmember which may cause less abrasion of a hot-press forming die duringhot-press forming, and a method of manufacturing the same.

The purpose of the present disclosure is not limited to the abovedescription. A person skilled in the art to which the present disclosurebelongs will not have any difficulty in understanding an additionalpurpose of the present disclosure from the general matters in thepresent specification.

Technical Solution

An aspect of the present disclosure relates to a hot-press formed memberincluding a base steel sheet; and an aluminum alloy plated layer formedon the base steel sheet, wherein the aluminum alloy plated layerincludes an alloying layer (I) formed on the base steel sheet andincluding, by weight %, 5-30% of Al; an alloying layer (II) formed onthe alloying layer (I) and including, by weight %, 30 to 60% of Al; analloying layer (III) formed on the alloying layer (II) and including, byweight %, 20-50% of Al and 5-20% of Si; and an alloying layer (IV)formed continuously or discontinuously on at least a portion of asurface of the alloying layer (III) and including 30-60% of Al, andwherein a rate of the alloying layer (III) exposed on an outermostsurface of the aluminum alloy plated layer is 10% or more.

A plurality of pores may be formed in the alloying layer (III), andporosity of the alloying layer (III) may be 5-50%.

The base steel sheet may include, by weight %, 0.04-0.5% of C, 0.01-2%of Si, 0.1-5% of Mn, 0.001-0.05% of P, 0.0001-0.02% of S, 0.001-1% ofAl, 0.001-0.02% of N, and a balance Fe and other impurities.

The base steel sheet may further include, by weight %, one or more of0.001-0.01% of B, 0.01-1% of Cr, and 0.001-0.2% of Ti.

Another aspect of the present disclosure relates to a method ofmanufacturing a hot-press formed member, the method includingaluminum-plating a surface of a base steel sheet and coiling the steelsheet, thereby obtaining an aluminum-plated steel sheet; annealing thealuminum-plated steel sheet, thereby obtaining an aluminum-ironalloy-plated steel sheet; and hot-press forming the aluminum-iron alloyplated steel sheet, wherein an aluminum plating amount is 30-200 g/m²based on a single side surface of the steel sheet, wherein a coolingrate to 250° C. is less than 20° C./sec after the aluminum plating,wherein coiling tension is 0.5-5 kg/mm² during the coiling, wherein theannealing is performed in a batch annealing furnace in a heatingtemperature range of 550-750° C. for 30 minutes-50 hours, wherein, whenthe heating is performed from room temperature to the heatingtemperature during the annealing, an average temperature increase rateis 10-100° C./h, and an average temperature increase rate in a 400-500°C. range is 1-15° C./h, wherein a difference between an atmospheretemperature in the batch annealing furnace and a temperature of thesteel sheet is determined to be 5-80° C., and wherein a heat treatmentis performed in a temperature range of Ac3-950° C. during the hot-pressforming, the heating is performed at a temperature increase rate of3-18° C./s to a temperature range from 200° C. to Ac3-950° C., and aheat treatment is performed for 1-15 minutes as a total heating time,and hot-press forming is performed.

An average abrasion depth of 10 points of a hot-press forming die may be15 μm or less when the hot-press formed member is produced 500 times bythe method of manufacturing the hot-press formed member.

Advantageous Effects

According to the present disclosure, when a hot-press formed member ismanufactured, surface hardness of a plated layer may be lower than thatof a hot-press forming die, such that abrasion of the die may reduce,thereby increasing grinding or replacement cycle of the hot-pressforming die, and improving manufacturing costs and production efficiencyof the hot-press formed member.

Various and beneficial advantages and effects of the present disclosureare not limited to the above description, and will be more easilyunderstood in the course of describing specific embodiments of thepresent disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an image of a cross-sectional surface of a plated layer of ahot-press formed member manufactured by inventive example 1, obtainedusing a scanning electron microscope; and

FIG. 2 is an image of a cross-sectional surface of a plated layer of ahot-press formed member manufactured by comparative example 1, obtainedusing a scanning electron microscope.

BEST MODE FOR INVENTION

In the description below, a hot-press formed member will be described indetail according to an aspect of the present disclosure. In the presentdisclosure, it is noted that a content of each element may indicate byweight % unless otherwise indicated. Also, a ratio of crystals orstructures may be based on an area unless otherwise indicated.

[Hot-Press Formed Member]

A hot-press formed member may include a base steel sheet, and analuminum alloy plated layer formed on the base steel sheet, and thealuminum alloy plated layer may include an alloying layer (I) formed onthe base steel sheet and including, by weight %, 5-30% of Al; analloying layer (II) formed on the alloying layer (I) and including, byweight %, 30 to 60% of Al; an alloying layer (III) formed on thealloying layer (II) and including, by weight %, 20-50% of Al and 5-20%of Si; and an alloying layer (IV) formed continuously or discontinuouslyon at least a portion of a surface of the alloying layer (III) andincluding 30-60% of Al.

Preferably, each of the alloy layers may have component ranges as below.The alloying layer (I) may include, by weight %, 5-30% of Al, 0-10% ofSi, and a balance of Fe and other inevitable impurities included byalloying, the alloying layer (II) may include, by weight %, 30-60% ofAl, 0-5% of Si, and a balance of Fe and other inevitable impuritiesincluded by alloying, the alloying layer (III) may include, by weight %,20-50% of Al, 5-20% of Si, and a balance of Fe and other inevitableimpurities included by alloying, and the alloying layer (IV) mayinclude, by weight %, 30-60% of Al, 0-5% of Si, and a balance of Fe andother inevitable impurities included by alloying.

When the base steel sheet is aluminum-plated and a heat treatment isperformed thereon, Fe of the base steel sheet may be diffused to thealuminum plated layer having a high Al content. In the hot-press formedmember in the present disclosure, alloying between Al and Fe may occurin the plated layer through an annealing treatment for alloying and aheat treatment during hot-press forming, and a layer structureconsisting of the alloying layers (I)-(IV) may be formed depending onthe degree of alloying of Fe.

The alloying layer (IV) may be formed continuously or discontinuously onat least a portion of the surface of the alloying layer (III). That is,the alloying layer (IV) may be formed on a portion of the surface of thealloying layer (III), rather than formed on the entire surface thereof.

Also, as the alloying layer (IV) is formed on at least a portion of thesurface of the alloying layer (III), a portion of the surface of thealloying layer (III) may be exposed to an outermost surface of thealuminum alloy plated layer. Here, the outermost surface may refer to anoutermost surface of the aluminum alloy plated layer on an opposite sideof the base steel sheet. When an oxide layer is formed on the surface ofthe aluminum alloy plated layer, the outermost layer may refer to anuppermost surface of the layers other than the oxide layer.

In this case, a ratio of the alloying layer (III) exposed on theoutermost surface of the aluminum alloy plated layer may be preferably10% or more. Here, the ratio of the alloying layer (III) exposed on theoutermost surface may be defined as the ratio of the length of an areain which the alloying layer (III) is exposed to a total length of theoutermost surface when observing a cross-sectional surface of the alloyplated layer, or, in some cases, the ratio may be defined as an arearatio of the surface area of the alloying layer (III) exposed on theoutermost surface to a surface area of the outermost surface of thealuminum alloy plated layer. Among the alloying layers, hardness of thealloying layer (II) and the alloying layer (IV) may be extremely high,about 900 Hv, while hardness of the alloying layer (I) and the alloyinglayer (III) may be about 300-700 Hv, relatively lower than those of thealloying layer (I) and the alloying layer (III). Therefore, when theexposed area of the alloying layer (III) having relatively low hardnessincreases on the outermost surface of the aluminum alloy plated layer incontact with the die during hot-press forming, overall average hardnessof the outermost surface may decrease, such that abrasion of the die maydecrease.

When the ratio of the alloying layer (III) exposed to the outermostsurface is less than 10%, a difference between average hardness of theoutermost surface and die hardness may decrease, such that die abrasionmay not be effectively prevented. In terms of inhibiting die abrasion,the lower the hardness of the outermost surface of the aluminum alloyplated layer, the more preferable it may be, and thus, it may not benecessary to limit an upper limit of the ratio. Preferably, the ratiomay be 15% or more, and in some cases, 20% or more.

A plurality of pores may be formed in the alloying layer III. When analuminum alloy-plated steel sheet is manufactured by performing analloying heat treatment on the aluminum-plated steel sheet in a batchannealing furnace under predetermined conditions, a plurality of alloylayers may be formed in the aluminum alloy-plated steel sheet, and aplurality of pores may be formed on an upper end alloy layer due to adifference in mutual diffusion coefficients such as Fe, Al, and Sibetween the alloy layers having different components. In this case, asan increasing number of pores are formed toward the upper end of thealloy layer, porosity may be high therein, and when the aluminum alloyplated steel sheet is heated and hot-press formed, an uppermost alloylayer having pores in high density may be broken to be small particlesby press forming, and since rolling friction occurring when the smallparticles rolls may be smaller than sliding friction between the steelsheet and the die, lubricity between the die and the steel sheet mayincrease. The porosity may be defined as a ratio of the pore area to thearea of each alloy layer (or the alloying layer) when thecross-sectional surface of the alloy layer (or the alloying layer) isobserved.

However, as in FIG. 1 , since most of the areas of the alloying layer IVare broken by the press forming during the hot-press forming, it may bedifficult to measure porosity of the alloying layer IV in the hot-pressformed member. Thus, the features of the present disclosure may berevealed through porosity of the alloying layer (III), which is lessaffected by press forming and has a close relationship with porosity ofthe alloying layer (IV) before press forming.

Accordingly, porosity of the alloying layer (III) of the hot-pressformed member according to an aspect of the present disclosure may be5-50%. When the porosity is less than 5%, it is difficult to expect alubrication effect by a rolling friction effect during the hot-pressforming. When the porosity exceeds 50%, the structure of the alloyinglayer (III) of the hot-press formed member may be excessively weakened,such that die contamination caused by particles fallen out of the platedlayer in the die may increase during continuous hot-press forming.Therefore, in the present disclosure, the porosity may be preferably5-50%, and in some cases, 7-50%.

The base steel sheet of the present disclosure may be a steel sheet forhot-press forming, and when the base steel sheet is used for hot-pressforming, a composition thereof may not be particularly limited. However,according to an aspect of the present disclosure, the base steel sheetmay include, by weight %, 0.04-0.5% of C, 0.01-2% of Si, 0.1-5% of Mn,0.001-0.05% of P, 0.0001-0.02% of S, 0.001-1% of Al, 0.001-0.02% of N,and a balance Fe and other impurities. Each composition system will bedescribed in greater detail.

C: 0.04-0.5%

C may be an essential element to increase strength of a heat treatmentmember, and may be added in an appropriate amount. That is, to securesufficient strength of the heat treatment member, C may be added by0.04% or more. Preferably, a lower limit of C content may be 0.1% ormore. However, when the content is too high, when the cold rolledmaterial is produced, strength of the hot rolled material may be toohigh when the hot rolled material is cold rolled, such that cold rollingproperties may greatly degrade and spot weldability may greatly degrade.Thus, to secure sufficient cold rolling properties and weldability, Cmay be added in an amount of 0.5% or less. Also, the C content may be0.45% or less, and more preferably, the content may be limited to 0.4%or less.

Si: 0.01-2%

Si may be added as a deoxidizer in steel making, and may also inhibitformation of carbides, which may have the greatest effect on strength ofthe hot-press-formed member. In the present disclosure, in the hot-pressforming, to secure retained austenite by concentrating carbon onmartensite lath grain boundaries after martensite is formed, the Sicontent may be 0.01% or more. Also, when aluminum plating is performedon the steel sheet after rolling, an upper limit of the Si content maybe determined to be 2% to ensure sufficient plating properties.Preferably, the Si content may be limited to 1.5% or less.

Mn: 0.1-5%

Mn may be added in an amount of 0.1% or more to secure a solid solutionstrengthening effect and also to reduce a critical cooling rate forsecuring martensite in the hot-press-formed member. Also, as Mn mayappropriately maintain strength of the steel sheet, Mn may securehot-press forming process workability, may reduce manufacturing costs,and may improve spot weldability, and thus, the Mn content may belimited to 5% or less.

P: 0.001-0.05%

P may be present as one of impurities in steel, and it may be moreadvantageous when the content thereof is low. Therefore, in the presentdisclosure, the P content may be limited to 0.05% or less, andpreferably, to 0.03% or less. As the smaller the amount of P, the moreadvantageous it may be as one of impurity elements, it may not benecessary to limit an upper limit thereof. However, to excessively lowerthe P content, manufacturing costs may increase, and thus, inconsideration thereof, a lower limit thereof may be determined to be0.001%.

S: 0.0001-0.02%

S may be one of impurities in steel, and may impair ductility, impactproperties, and weldability of the member, and thus, a maximum contentmay be limited to 0.02%, and preferably to 0.01% or less. Also, when aminimum content is less than 0.0001%, manufacturing costs may increase,and thus, a lower limit of the content may be determined to be 0.0001%.

Al: 0.001-1%

Al may increase cleanliness of steel by deoxidizing in steelmakingtogether with Si, and may be added in an amount of 0.001% or more toobtain the above effect. Also, the content of Al may be limited to 1% orless to prevent an Ac3 temperature from excessively increasing such thatheating required during the hot-press forming may be performed within anappropriate temperature range.

N: 0.001-0.02%

N may be included as one of impurities in steel, and to reducesensitivity against cracking during slab continuous casting and tosecure impact properties, a lower content thereof may be advantageous,and thus, N may be added by 0.02% or less. It may be necessary todetermine a lower limit, but in consideration of an increase inmanufacturing costs, the N content may be determined to be 0.001% ormore.

In addition to the alloy composition described above, the aluminum-ironalloy plated steel sheet according to an aspect of the presentdisclosure may include one or more of 0.001-0.01% of B, 0.01-1% of Cr,and 0.001-0.2% of Ti.

B: 0.001-0.01%

B may improve hardenability even with a small amount, and may besegregated on a prior austenite grain boundary such that B may inhibitbrittleness of the hot-press-formed member by grain boundary segregationof P and/or S. Therefore, 0.0001% or more of B may be added. When thecontent exceeds 0.01%, the effect may be saturated, and brittleness maybe caused in hot rolling, and thus, an upper limit thereof may bedetermined to be 0.01%, and preferably, the B content may be determinedto be 0.005% or less.

Cr: 0.01-1%

Cr may be added to obtain a solid solution strengthening effect and toimprove hardenability during hot-press forming similarly to Mn, and0.01% or more of Cr may be added to obtain the above effect. To secureweldability of the member, however, the content may be limited to 1% orless, and when the content exceeds 1%, the effect of improvinghardenability may be insignificant as compared to the added amount,which may be disadvantageous in terms of costs.

Ti: 0.001-0.2%

Ti may be effective in improving strength of the heat treatment memberby forming fine precipitates and improving collision performance of themember by refinement of grains, and also, when B is added, B may bepreferentially reacted with N such that the effect of adding B may bemaximized. To obtain the above effect, Ti may be added in an amount of0.001% or more. However, the formation of coarse TiN caused by anincrease in the Ti content may deteriorate the collision performance ofthe member, and thus, the content may be limited to 0.2% or less.

A remainder other than the above-described components may include iron(Fe) and inevitable impurities, and addition of any element may not beparticularly limited as long as the element is able to be included inthe steel sheet for the hot-press forming.

When the hot-press formed member having the above-described alloycomposition and the layer structure is manufactured, a ratio of thealloying layer (III) having low hardness may increase on the surface ofthe aluminum alloy plated steel sheet during hot-press forming, suchthat average hardness of the surface may decrease, and thus, abrasion ofthe die caused by a difference in hardness may be effectively reduced.In particular, even when the hot-press formed member is produced morethan 500 times, the 10-point average abrasion depth of the hot-pressforming die may be 15 μm or less.

Also, a plurality of pores may be formed in the alloying layers (III)and (IV), upper end layers of the aluminum alloy plated layer, duringthe hot-press forming, and the alloying layer (IV) may be broken due tothe pores during the press forming, such that a lubrication effect byrolling friction may be obtained, and damage to the die may further beprevented.

Hereinafter, a method of manufacturing a hot-press formed memberaccording to another aspect of the present disclosure will be describedin detail. The method of manufacturing a hot-press formed memberdescribed below may be merely an example, and it does not indicate thatthe hot-press formed member in the present disclosure needs to bemanufactured by the manufacturing method. Any manufacturing method whichmay satisfy claims of the present disclosure may have no problem inimplementing each embodiment of the present disclosure.

[Method of Manufacturing Hot-Press Formed Member]

The hot-press formed member in the present disclosure may be obtained bypreparing a hot-rolled or cold-rolled base steel sheet, performingaluminum plating on a surface of the base steel sheet, performing analloying heat treatment in a batch annealing furnace to obtain analuminum alloy plated steel sheet, and performing hot-press formingunder predetermined conditions.

First, a process of preparing the base steel sheet having theabove-described alloy composition, plating aluminum on the surface ofthe base steel sheet under appropriate conditions, and coiling the steelsheet, thereby obtaining an aluminum plated steel sheet (coil) may beperformed.

An aluminum plating treatment may be performed on the surface of therolled steel sheet in a plating amount of 30-200 g/m² based on a singleside. The aluminum plating may be AlSi plating (including 80% or more ofAl and 5-20% of Si, and additional elements may be included ifnecessary), generally known as type I, and plating requiring 90% or moreof Al and including additional elements if necessary, which may be knownas type II, may also be used. To form a plated layer, hot-dip aluminumplating may be performed, and an annealing treatment may be performed onthe steel sheet before plating. In the plating, a proper amount ofplating may be 30-200 g/m² based on a single side. When the amount ofplating is excessively high, it may take an excessive amount of time toalloy to the surface, and when the amount of plating is extremely low,it may be difficult to obtain sufficient corrosion resistance.Thereafter, after the aluminum plating, cooling may be performed at acooling rate of 20° C./sec or less to 250° C. The cooling rate after thealuminum plating may affect the formation of the diffusion inhibitinglayer between the plated layer and the base iron. When the cooling rateafter aluminum plating is extremely high, the diffusion inhibiting layermay not be uniformly formed, and alloying behavior of a coil during anannealing treatment subsequently performed may become uneven. Therefore,the cooling rate to 250° C. after the aluminum plating may be determinedto be 20° C./sec or less.

When a coil is obtained by coiling the steel sheet after the plating,coiling tension of the coil may be adjusted to be 0.5-5 kg/mm².Depending on the adjustment of the coiling tension of the coil, alloyingbehavior and surface quality of the coil may differ during the annealingtreatment subsequently performed.

Thereafter, the aluminum-plated steel sheet may be obtained byperforming the annealing treatment under the conditions as below,thereby obtaining an aluminum-iron alloy plated steel sheet.

The aluminum-plated steel sheet (coil) may be heated in a batchannealing furnace (BAF). When the steel sheet is heated, as for the heattreatment target temperature and the maintaining time, the steel sheetmay be maintained for 30 minutes-50 hours within a range of 550-750° C.(in the present disclosure, a highest temperature that the materialreaches in this temperature range may be referred to as a heatingtemperature) based on the steel sheet temperature preferably. Themaintaining time may be the time until the cooling is initiated afterthe coil temperature reaches the target temperature. When the alloyingis not sufficiently performed, the plated layer may be peeled off duringthe roll leveling, and thus, the heating temperature may be determinedto be 550° C. or higher for sufficient alloying. Also, to preventexcessive formation of oxides on the surface layer and to secure spotweldability, the heating temperature may be 750° C. or less. Also, tosufficiently secure the plated layer and to prevent degradation inproductivity, the maintaining time may be determined to be 30 minutes-50hours. In some cases, the temperature of the steel sheet may have aheating pattern in which the temperature may continue to rise without acooling process until the heating temperature is reached, or a heatingpattern in which the temperature equal to or lower than the targettemperature may be maintained for predetermined time and may rise.

When the steel sheet is heated at the above-described heatingtemperature, to ensure sufficient productivity and to uniformly alloythe plated layer in the entire steel sheet (coil), an averagetemperature increase rate may be 10-100° C./h with reference to thesteel sheet (coil) temperature for the entire temperature range (therange from room temperature to the heating temperature). The overallaverage temperature increase rate may be controlled within the abovenumerical range, but in an embodiment of the present disclosure, toprevent surface stains caused by a rolling oil remaining in thetemperature range in which the rolling oil mixed into during rolling isvaporized and to secure sufficient productivity, the heating may beperformed with an average temperature increase rate in the range of400-500° C. to be 1-15° C./h when the temperature rises.

Also, a difference between an atmosphere temperature and the temperatureof the steel sheet in the batch annealing furnace may be 5-80° C.Generally, the heating of the batch annealing furnace may be performedby a method of heating the steel sheet (coil) through an increase in theatmosphere temperature in the annealing furnace, rather than a method ofdirectly heating the steel sheet (coil). In this case, the differencebetween the atmosphere temperature and the coil temperature may not beavoided, but to minimize a difference in materials and plating qualitiesfor each position in the steel sheet, a difference between theatmosphere temperature and the steel sheet temperature may be 80° C. orless based on the time point at which the heat treatment targettemperature is reached. It may be ideal to reduce the temperaturedifference as much as possible, but the decreased temperature differencemay decrease the temperature increase rate, such that it may bedifficult to satisfy the entire average temperature increase ratecondition, and thus, considering this, the difference may be determinedto be 5° C. or more. The temperature of the steel sheet may refer to thetemperature measured from a bottom portion (a lowest portion of thecoil) of the charged steel sheet (coil), and the atmosphere temperaturemay refer to the temperature measured at a center of an internal spaceof the heating furnace.

After the aluminum alloy plated steel sheet is manufactured by theabove-described manufacturing method, a hot-press forming may beperformed on the aluminum alloy plated steel sheet, therebymanufacturing a hot-press formed member. In this case, as the hot-pressforming, a method generally used in the technical field may be used, andas an example, although not limited thereto, a heat treatment may beperformed in a temperature range of Ac3-950° C., heating may beperformed at a temperature increase rate of 3-18° C./s from 200° C. to atemperature range of Ac3-950° C., a heat treatment may be performed for1-15 minutes as a total heating time, and hot-press forming may beperformed. The total heating time may be defined as the heating timeincluding both the heating time in the temperature increase range andthe heating time in the temperature range of Ac3-950° C.

BEST MODE FOR INVENTION

Hereinafter, the present disclosure will be described in greater detailthrough embodiments. However, it should be noted that the embodiment aremerely to specify the present disclosure and not to limiting the scopeof the present disclosure. The scope of the present disclosure may bedetermined by matters described in the claims and matters reasonablyinferred therefrom.

Embodiment

First, a cold-rolled steel sheet for hot-press forming having thecomposition as in Table 1 was prepared as a base steel sheet, and thesurface of the steel sheet was plated with a type I plating bath havingan Al-9% Si-1.5% Fe composition. During the plating, the plating amountwas adjusted to be 75 g/m² per a single side, and cooling was performedat a cooling rate of 10° C./sec to 250° C. after the aluminum plating,and coiling tension was adjusted to be 3 kg/mm², thereby obtaining analuminum-plated steel sheet.

TABLE 1 Element C Si Mn Al P S N Cr Ti B Ac3 Content 0.23 0.2 1.25 0.030.01 0.002 0.005 0.21 0.034 0.0022 822° C. (%)

Thereafter, an alloying heat treatment was performed on the plated steelsheet in a batch annealing furnace under the conditions as in Table 2below, and hot-press forming was performed 500 times on each sample,thereby obtaining a hot-press formed member. However, in comparativeexample 1, the alloying heat treatment was not performed on theabove-described aluminum-plated steel sheet, and hot-press forming wasperformed under the conditions as in Table 2, thereby obtaining ahot-press formed member.

TABLE 2 Alloying heat treatment conditions Av- Tem- erage pera- tem-ture pera- differ- ture ence in- be- crease tween Av- rate in atmo-Hot-press forming erage 400- sphere conditions tem- 500 and Tem- pera- °C. steel pera- ture tem- sheet at ture in- pera- heating in- Total Tem-crease ture tem- crease Tem- heat- Classi- pera- rate range pera- ratepera- ing fica- ture (° C./ (^(o) C. ture Time (° C. ture time tion (°C.) h) /h) (° C.) (h) /s) (° C.) (min) Inven- 630 21  6 25 14 5.8 930 5tive exam- ple l Inven- 590 25 10 30 30 8.5 900 6 tive exam- ple 2Inven- 680 27 12 25  8 6.2 930 5 tive exam- ple 3 Com- — — — — — 3.4 9305 para- tive exam- ple l Com- 500 35 20 25  8 4.7 900 6 para- tive exam-ple 2

Thereafter, for the die used in each inventive example and comparativeexample, the member was produced 500 times, an abrasion depth wasmeasured at random 10 points, and average values thereof are listed inTable 3 below. Among 500 products for each example, random ten sampleswere taken, cross-sectional surfaces thereof were observed using ascanning electron microscope to confirm occupancy of the outermost layerof the alloying layer (III), and average values of the occupancy arelisted in Table 3 below. Also, porosity of the alloying layer (III) wasmeasured, and results thereof are listed in Table 3 below. In the sameembodiment (inventive example or comparative example), it has beenindicated that a deviation between the occupancy in the outermost layerand porosity of the alloying layer (III) was not large.

TABLE 3 Ratio of alloying Average die Porosity of layer (III) exposedabrasion depth alloying on outermost after 500 layer Classificationsurface (%) productions (μm) (III) (%) Inventive 35 7 7.2 example 1Inventive 12 9 5.7 example 2 Inventive 47 4 13.1  example 3 Comparative 7 39  1.7 example 1 Comparative  9 23  3.2 example 2

As indicated in Table 3, as for inventive examples 1 to 3 in which anarea ratio of the alloying layer (III) exposed on the outermost surfacewas 10% or more, and the porosity is 5% or more, it has been confirmedthat, even when the hot-press formed member was produced 500 timesaccording to inventive examples 1 to 3, an average abrasion depth of thedie was 15 μm or less, such that abrasion of the hot-press forming diewas effectively inhibited. As for comparative example 1, a general Al—Siplated steel sheet was hot-press formed, and the area ratio of thealloying layer (III) exposed on the outermost surface was less than 10%,and the porosity was low, such that abrasion of the die greatlyincreased as compared to the invention example.

Also, as for comparative example 2, the alloying heat treatment of thealuminum plated layer was performed, but the alloying heat treatmenttemperature was low, such that the alloying was not sufficientlyperformed. Accordingly, it has confirmed that, as the area ratio of thealloying layer (III) exposed on the outermost surface was less than 10%,and the porosity was low, abrasion of the die significantly increased asin comparative example 1.

While the example embodiments have been illustrated and described above,it will be apparent to those skilled in the art that modifications andvariations could be made without departing from the scope in the exampleembodiment as defined by the appended claims.

The invention claimed is:
 1. A hot-press formed member, comprising: abase steel sheet; and an aluminum alloy plated layer formed on the basesteel sheet, wherein the aluminum alloy plated layer includes: analloying layer (I) formed on the base steel sheet and including, byweight %, 5-30% of Al, 0-10% of Si, and a balance of Fe and otherinevitable impurities; an alloying layer (II) formed on the alloyinglayer (I) and including, by weight %, 30 to 60% of Al, 0-5% of Si, and abalance of Fe and other inevitable impurities; an alloying layer (III)formed on the alloying layer (II) and including, by weight %, 20-50% ofAl and 5-20% of Si, and a balance of Fe and other inevitable impurities;and an alloying layer (IV) formed continuously or discontinuously on atleast a portion of a surface of the alloying layer (III) and including30-60% of Al, 0-5% of Si, and a balance of Fe and other inevitableimpurities, and wherein a ratio of the alloying layer (III) exposed onan outermost surface of the aluminum alloy plated layer is 10% or more.2. The hot-press formed member of claim 1, wherein a plurality of poresare formed in the alloying layer (III), and wherein porosity of thealloying layer (III) is 5-50%.
 3. The hot-press formed member of claim1, wherein the base steel sheet includes, by weight %, 0.04-0.5% of C,0.01-2% of Si, 0.1-5% of Mn, 0.001-0.05% of P, 0.0001-0.02% of S,0.001-1% of Al, 0.001-0.02% of N, and a balance Fe and other impurities.4. The hot-press formed member of claim 3, wherein the base steel sheetfurther includes, by weight %, one or more of 0.001-0.01% of B, 0.01-1%of Cr, and 0.001-0.2% of Ti.